Walls about 10 m high, a deep ditch dug in front of the wall, and about 100 fortress towers ensured a reliable defense. The most difficult and controversial stage in the history of Nicaea, like the whole of Asia Minor, came in the 11th – 15th centuries, when the region went through a painful transformation from Greek to Turkish, and most of the inhabitants completely changed their language, religion and self-consciousness. Until 1922, the Assumption Church decorated with mosaics remained intact - the most significant monument of Byzantine art in Asia Minor. In 1922 it was destroyed by the Turkish authorities. Today, the medieval wall is also destroyed in many places.

The name of the first city on the place of Nikaia was Ankore or Helicore, this city was destroyed by the Mizians. A few years after the death of Alexander the Great, the Macedonian king Antigonus I the One-eyed (who controlled Asia Minor after the death of Alexander), possibly after the victory over Eumenes in 316 BC., restored the city and called it Antigonia. Several former commanders of Alexander (known as the “diadochi”) united against Antigone, and after the defeat of Antigone the region came under control Thracian warlord Lysimachus in 301 BC, which renamed the city of Nicea (Greek: Νίκαια) in honor of his wife Nikaia, the daughter of Antipater.

The city was built in the form of a square with a side of 16 stades; the gates of the city were four; all the streets of Nicea intersected each other at a right angle so that all the gates could be seen from the geometric center of the city where the monument was erected. This monument stood in the gymnasium, destroyed by a fire, but restored by Pliny the Younger when he was governor of Bithynia.

The city was built at the intersection of important trade routes between Galatia and Phrygia and led successful trade. Over time, the value of Nicaea increased, and the kings of Bithynia moved here, the era of which begins from 288 BC. starting with Zipoit, who spent a lot of time in Nicaea.

Strabo calls at this time Nicea the capital of Bithynia, which is confirmed by the inscriptions on some coins, but it is known that the capital later moved to Nicomedia (modern Izmit). For a long time these two cities disputed the right to be the capital. Pliny the Younger often mentions Nicaea and her public buildings, which he restored when he was the governor of Bithynia.

The following were born in Nicea: astronomer Hipparch (circa 194 BC ), historian Dion Cassius (circa 165), as well as mathematician and astronomer Spore.

Nicene coins indicate the interest of the city of emperors, often in the city celebrations were held in honor of gods and emperors. During the Roman period, Nicea remained an important city, being only 70 km from Constantinople. When Constantinople became the capital of the Eastern Roman Empire, Nicea did not lose its significance; moreover, at that time new walls were built and the old walls were strengthened, in the early 300s AD the construction of public and religious buildings was going on actively.

Nicea was hit by earthquakes in 358, 362 and 368; after the last earthquake, the city was restored by the emperor Valens. In the Middle Ages, Nicea fortress was strategically important during the confrontation of Byzantium and the Seljuk Turks. For a long time, Nicea was ruled by Byzantium, but in 1077 the Seljuk Turks seized the city. Several times Nicaea passed from hand to hand, until finally in 1078 it came under the control of the Seljuk Turks. Nicea becomes the capital of the Rumsky sultanate.

These events prompted the First Crusade, and at the request of Byzantium, the Crusaders, with the support of smaller Byzantine units, approached the city in 1097. Nicea was besieged: the siege lasted for more than a month, after which the Turks chose to surrender to the Byzantine troops, not giving a chance to the participants of the crusade to plunder the city. To mitigate the dissatisfaction of the Crusaders, the Byzantine emperor Alexey I Komnenos provided the horses and transferred part of the ransom.

Constantinople fell in 1204 during the Fourth Crusade, after which it became the capital of the Latin Empire created by the crusaders.

After the fall of Constantinople, several other states were formed on the territory of Byzantium - the Epirus despoat, the Trapezund and Nicene empires, the first emperor of which was Theodore I Lascaris. Theodore I and his heirs expanded the territory of the state and increased their influence until, in 1259, Mikhail VIII Paleologus usurped the throne. He conquered Constantinople in 1261 and revived the Byzantine Empire. After that, Nicea lost its importance for Byzantium and soon became easy prey for the Ottoman Turks.

March 2, 1331 after a two-year siege and the defeat of the Byzantine "army" the city surrendered to the Ottoman Turks and was renamed Iznik.

The gradual transformation of the city from Greek to Turkish had a long and complex prehistory. The Byzantine Nicea fell for the first time in 1071, after the Battle of Manzikert won by the Seljuk Turks opened the gates to Asia Minor. It was then that the city first became the center of the Muslim Sultanate, although the resistance of the local Greek-Christian population, especially the military settlers of the acrites, was still great. So joint actions of the Byzantines and the Crusaders at the initial stage led to the liberation of the city and coastal regions from the Turks by 1097. And even after the betrayal of the Crusaders and Venice, who plundered Constantinople in 1204

Nicea was able to gather the necessary forces to fight back both knights from the West and Turks from the East. The city became the center of the new Greek state, called the Nicene Empire, which led an active policy of restoring Byzantium. At the same time, the hatred of the crusaders, who seized the capital of the former empire, eclipsed a sober look at more dangerous and real opponents - the Turks, with whom the Greeks initially began to openly cooperate. Turkish mercenaries settled all large regions of Asia Minor and, eventually, imperceptibly surrounded the city.

On February 23, 1265, the population of the city embraced the fear of the Mongol invasion. The panic was unreasonable, since the Mongols had always been allies of the Byzantines in the fight against the Turks. However, most wealthy citizens chose to move to Constantinople and Thrace. The constant outflow of the Christian population to Europe further undermined the city’s ability to resist.

On March 2, 1331, after a thirty-year blockade, the city came under the control of the Ottoman Empire. After the capture of the city by the Ottoman Turks, he entered the core of a young and aggressive Ottoman state. Ibn Battuta visited Nicaea in October 1331, seven months after the Ottoman conquest. The city seemed to him neglected and deserted. Many public buildings were destroyed, and building materials were used for mosques and other buildings, some of the temples were converted into mosques. In 1333, two years after the fall of Nicea, the emperor Andronicus bought books, relics and other preserved church plate and moved it from Nicea to Constantinople. The fall of Nicaea placed Andronicus in an extremely vulnerable position: he had to start paying tribute to the Ottomans for the preservation of Scutari, Nicomedia, Heraclia and Peg. But the delay was only temporary: in 1337, the Turks starved out Nicomedia.

The Patriarch of Constantinople was shocked when he learned that after 7 years, in 1338, most of the once demoralized Greek residents of the city had already converted to Islam and spoke Turkish well. His letters to the remaining residents with calls to adhere to the Christian faith were never heard. In 1331–1365 (until the fall of another Greek city - Adrianople (Edirne)) - the city is the capital of the Ottoman Turks. After the fall of Constantinople in 1453, the city ​​lost its importance, but later (in the XVII century) became the main center for the production of tableware (known as İznik Çini - translated from Turkish Iznik's China, Chinese porcelain was the favourite dishes of the sultans). Iznik tiles are decorated with many Istanbul mosques. Over time, production moved to Istanbul, and Iznik became a small agricultural center, aided by the lack of rail links.

Today, dishes are traditionally produced in İznik, but the center of local production is located in the town of Kutahya.

During the reign of the Roman emperor Constantine, in 325, the first Ecumenical Council in the history of Christianity was held in Nicea. At the Council, the Symbol of Faith was adopted, which affirmed the dogma of the Trinity and formed the basis of orthodox Christianity, and the Arian heresy was condemned. Also at the Cathedral was determined the time of the celebration of Easter. This concept of the Trinity was supplemented and expanded at the second Ecumenical Council in Constantinople in 381.

Hagia Sophia was built by Justinian I in the center of the city in the 6th century (as a reduced copy of Aya Sophia in Constantinople), and it was there that the Second Council of Nicaea was held in 787 to discuss the iconoclasm problem.

Iznik remains the nominal bishopric of the Roman Catholic Church, a place free from the death of its last bishop in 1976.

The ancient walls with towers and gates are relatively well preserved. The thickness of the walls is 5 to 7 m, the height is 10 to 13 m. The walls are made of large slabs of the Roman period and square stones fastened with cement. The city could be reached through 4 large and 2 small gates. In some places you can see the columns, other architectural fragments, the remains of ancient buildings. The walls of Nicaea resemble the walls of Constantinople. On some towers you can see the Greek inscriptions.

Inside the Greek fortifications are the ruins of mosques, baths, Greek temples and churches, and other buildings.

Outside the city walls are visible remains of an aqueduct.

In the northwestern part of the city, two long breakwaters formed a harbor, but now the lake has receded, and in this part of the city there is a marshy plain.